System and method for downlink channel sounding in wireless communications systems

ABSTRACT

In accordance with an embodiment, a method of operating a base station configured to communicate with at least one user device includes transmitting a reference signal to the at least one user device, receiving channel quality information from the at least one user device, and forming a beam based on the channel quality information received from the at least one user device.

This patent application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 14/874,190 filed on Oct. 2, 2015, entitled “Systemand Method for Downlink Channel Sounding in Wireless CommunicationsSystems,” which is a continuation of U.S. Non-Provisional patentapplication Ser. No. 12/830,983 filed on Jul. 6, 2010, entitled “Systemand Method for Downlink Channel Sounding in Wireless CommunicationsSystems,” which claims priority to U.S. Provisional Application No.61/224,737 filed on Jul. 10, 2009, entitled “System and Method forDownlink Channel Sounding in Wireless Communications Systems,” all ofwhich are incorporated by reference herein as if reproduced in theirentireties.

TECHNICAL FIELD

The present invention relates generally to wireless communications, andmore particularly to a system and method for downlink (DL) channelsounding in wireless communications systems.

BACKGROUND

Wireless communication systems are widely used to provide voice and dataservices for multiple users using a variety of access terminals such ascellular telephones, laptop computers and various multimedia devices.Such communications systems can encompass local area networks, such asIEEE 801.11 networks, cellular telephone and/or mobile broadbandnetworks. The communication system can use a one or more multiple accesstechniques, such as Frequency Division Multiple Access (FDMA), TimeDivision Multiple Access (TDMA), Code Division Multiple Access (CDMA),Orthogonal Frequency Division Multiple Access (OFDMA), Single CarrierFrequency Division Multiple Access (SC-FDMA) and others. Mobilebroadband networks can conform to a number of system types orpartnerships such as, General Packet Radio Service (GPRS),3rd-Generation standards (3G), Worldwide Interoperability for MicrowaveAccess (WiMAX), Universal Mobile Telecommunications System (UMTS), the3rd Generation Partnership Project (3GPP), Evolution-Data OptimizedEV-DO, or Long Term Evolution (LTE).

In coordinated multi-point (CoMP) transmission, transmissions frommultiple enhanced Node Bs (eNBs) are made simultaneously to a singleUser Equipment (UE) or to multiple UEs. Coordination of thetransmissions made by the eNBs enable the UE to combine thetransmissions to improve high data rate coverage and to increase systemthroughput in advanced wireless communications systems, such as LongTerm Evolution-Advanced (LTE-A). eNBs are also commonly referred to asbase stations, base transceiver stations, controllers, access points,and so forth, while UEs are commonly referred to as subscribers,subscriber stations, terminals, mobile stations, for example.

There are generally two CoMP approaches: joint processing from multiplecells (eNBs) and coordinated scheduling/beamforming (CS/CB). In jointprocessing, there is an assumption that data is available at eachtransmission point (eNB) in a CoMP cooperating set representing eNBsparticipating in the CoMP transmission. With joint processing, data istransmitted from more than one eNB at a time. Dynamic cell eNB selectionoccurs, on the other hand, when the data is transmitted from only oneeNB at a time. In CS/CB, the data is available at a serving eNB, andtransmission scheduling is coordinated among eNBs within the CoMPcooperating set.

To achieve better channel utilization and increase overall systemperformance, channel state/statistics/information about a downlink (DL)channel(s) between an eNB and a UE are provided by the UE to the eNB.The channel state/statistics/information provided by the UE enables theeNB to adjust its transmitter to more effectively make use of DL channelconditions.

In general, there are two types of channel state/statistics/informationfeedback schemes for LTE-A: explicit channelstate/statistics/information feedback and implicit channelstate/statistics/information feedback. In explicit feedback, an eNBdetermines the CoMP transmission processing matrix based on the whole ormajor part of the CoMP channel information. Better CoMP performance can,therefore, be obtained at the expense of a high feedback overhead. Withexplicit feedback, more information is provided to the eNB to give theeNB more flexibility in scheduling CoMP transmissions. If precoded DLreference signals are used, a selected CoMP transmission scheme can betransparent to the UE. However, uplink (UL) feedback overhead may behigh when instantaneous channel information feedback is required.

In implicit feedback, an eNB determines the CoMP transmission processingmatrix based on a precoding matrix indication (PMI)/rank indication (RI)recommended by UE. For non-coherent multi-point CoMP transmission, onlydisjoint PMI/RI information (or individual PMI for cells in CoMPcooperation set) is used. For coherent multi-point CoMP transmission,joint PMI/RI feedback containing individual PMI/RI information andadditional inter-cell spatial information or a single joint PMI/RIinformation is used. Usually joint PMI/RI feedback has a higher overheadthan disjoint PMI/RI feedback. With implicit feedback, the UE feeds backchannel information based on transmit or receive processing, and incursless feedback overhead. Implicit feedback may come at decreasedscheduling flexibility.

LTE-A is capable of supporting advanced forms of multiple input,multiple output (MIMO), such as higher-order single user MIMO (SU-MIMO)or multi-user MIMO (MU-MIMO). For example, precoding with more than four(4) transmit antennas can be utilized in SU-MIMO and MU-MIMO. Moreaccurate tuning of a transmit beam and/or closed-loop (CL) spatialchannel to support of a variety of antenna configurations andpropagation scenarios is desired to fully exploit the benefit of moreadvanced forms of MIMO.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a method of operating a base stationconfigured to communicate with at least one user device includestransmitting a reference signal to the at least one user device,receiving channel quality information from the at least one user device,and forming a beam based on the channel quality information receivedfrom the at least one user device.

In accordance with a further embodiment, a method for operating in acommunications network having a plurality of communications devicesincludes performing a coarse tuning with the plurality of communicationsdevices to achieve a first degree of spatial granularity; and performinga fine tuning with a subset of the plurality of communications devicesto achieve a second degree of spatial granularity.

In accordance with a further embodiment, a base station includes anantenna, a transmitter coupled to the antenna, a receiver configured toreceive channel quality information from at least one user device, and aprocessor calculating a beam direction for the least one user device,the beam direction based on the received channel quality information.The transmitter is configured to transmit a sounding signal to the atleast one user device.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a communications system;

FIG. 2a is a flow diagram of eNB operations in DL transparent channelsounding;

FIG. 2b is a flow diagram of UE operations in DL transparent channelsounding;

FIG. 3a is a diagram of iterative PCSRS based DL channel sounding inadvanced MIMO;

FIG. 3b is a diagram of iterative PCSRS based DL channel sounding inCoMP transmission;

FIG. 4a is a flow diagram of eNB operations in PCSRS based DL channelsounding in CoMP transmission;

FIG. 4b is a flow diagram of UE operations in PCSRS based DL channelsounding in CoMP transmission;

FIG. 5a is a flow diagram of eNB operations in PCSRS based differentialcodebook feedback;

FIG. 5b is a flow diagram of UE operations in PCSRS based differentialcodebook feedback;

FIG. 6 is a diagram of PCSRS based channel sounding for CS/CB;

FIG. 7a is a flow diagram of eNB operations in PCSRS based DL channelsounding for CS/CB;

FIG. 7b is a flow diagram of UE operations in PCSRS based DL channelsounding for CS/CB;

FIG. 8 illustrates a block diagram of an embodiment base station;

FIG. 9 illustrates a block diagram of an embodiment relay node; and

FIG. 10 illustrates a block diagram of an embodiment user device.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments are discussed in detail below.It should be appreciated, however, that the present invention providesmany applicable inventive concepts that can be embodied in a widevariety of specific contexts. The specific embodiments discussed aremerely illustrative of specific ways to make and use the invention, anddo not limit the scope of the invention.

The present invention will be described with respect to variousembodiments in a specific context, a system and method for downlinkchannel sounding in a wireless communication system. Embodiments of theinvention may also be applied to other types of communications systemsand networks.

FIG. 1 illustrates embodiment communications system 100. Communicationssystem 100 includes a number of eNBs, such as eNB 105 and eNB 106 thatmay be connected to a gateway (G/W) 107 over a wired backhaul. The eNBsserve a number of UEs, such as UE 110, UE 111, and UE 112. Transmissionsfrom the eNBs to the UEs may occur directly, such as from eNB 105 to UE110, or indirectly, such as through a relay node (RN), including RN 115,RN 116, RN 117, and RN 118. For example, an indirect transmission occursthrough a single RN, such as a transmission from eNB 105 to UE 111through RN 115, or through more than one RNs, such as a transmissionfrom eNB 106 to UE 112 through RN 116 and RN 118.

In an embodiment, a RN may be used to relay transmissions made by an eNBto a UE or a UE to an eNB. The use of the RN can increase the coveragearea of the eNB. As discussed previously, CoMP transmission has beenproposed to increase the coverage of a UE by allowing transmissions frommultiple eNBs to be made simultaneously to the UE, commonly referred toas an access link. However, in embodiments, CoMP transmissions may alsobe used to increase the coverage of a RN by allowing transmissions frommultiple eNBs or RNs to be made simultaneously to a the RN, commonlyreferred to as a backhaul link.

As shown in FIG. 1, a first backhaul CoMP transmission from eNB 105 andeNB 106 may be made to RN 115 (shown as backhaul CoMP hop-1 125) and asecond backhaul CoMP transmission from RN 115 and RN 116 may be made toRN 117 (shown as backhaul CoMP hop-2 126). While an access link CoMPtransmission from RN 117 and RN 118 may be made to UE 112 (shown asaccess CoMP 130). The system illustrated in FIG. 1 is one example ofmany embodiment network configurations. In alternative embodiments, anynumbers of UE, eNBs and RNs can be used in other configurations.

Traditionally, CoMP transmission has been considered for an access link,which is a wireless link between an eNB and a UE, or between a relaynode (RN) and a UE. However, in embodiments, CoMP transmission is alsoapplied to backhaul link, which is a wireless link between an eNB and aRN, to enhance the coverage of the RN.

In embodiment communications systems with RNs, layered CoMPtransmissions are applied to backhaul links as well as to access links.With a backhaul CoMP transmission, a RN receives and transmit data frommore than one eNB at a time. In an embodiment, this may occur as long asdata is available in more than one eNB to allow for joint or coordinatedtransmissions to a particular RN, thereby enabling an inexpensive RNdeployment. In further embodiments, backhaul CoMP transmission is alsoused to control inter-cell interference. With access link CoMPtransmission, a UE can receive and transmit data from and to more thanone RN or eNB, with the possibility of multiple access link CoMPtransmissions for multi-hop RNs.

In embodiments, both backhaul CoMP transmissions and access link CoMPtransmissions may be configured separately or jointly. Jointconfiguration is referred to concatenated CoMP transmission. Using aseparate configuration, different CoMP transmission technologies andfeedback schemes can be used in each CoMP transmission layer. In anembodiment, an RN can function both as a CoMP transmission transmitpoint and/or a CoMP transmission receive point.

In LTE-A, a UE specific demodulation reference signal is used fortransparent DL transmission. In an embodiment using a UE specificdemodulation reference signal, an eNB does not signal the transmissionmode/processing since the same transmission mode/processing is appliedto both the data and the UE specific demodulation reference signal. Thisallows the eNB greater freedom in the selection of the transmissionmode, for example, by giving the eNB more scheduling flexibility.

In order for the eNB to determine the transmission mode, enough channelinformation is made available to the eNB in order for the eNB to make adecision. With explicit channel (H) feedback, the eNB obtains theentirety of the channel information. However, the feedback overhead canbe high, especially for higher-order MIMO, MU-MIMO, and CoMPtransmission. With implicit channel feedback, such as PMI feedback, theeNB relies on the recommendation of the UE(s), which can restrict someof the scheduling available at the eNB.

In an embodiment, DL precoded common sounding reference signal (PCSRS)based channel sounding is applied to CoMP channel sounding. DL PCSRS iseNB oriented, with the eNB configuring the precoding matrix adaptivelyaccording to the deployment environment, such as antenna configuration,neighboring eNBs available for CoMP transmission, propagation scenario,UE distribution, and so forth. The sounding reference signal occupies DLchannel resources; therefore, less UL channel resources are required.Transparent DL channel sounding allows for the true transparentnon-codebook based precoding on the UE side with simple UEimplementation that does not require codebook searches or feedback.

FIG. 2a illustrates a flow diagram of embodiment DL transparent channelsounding method 200 for a base station, such as an eNB 105. In oneembodiment, method 200 is used to DL transparent channel sounding toobtain feedback information to schedule CoMP transmissions.

In step 205, the eNB transmits a common sounding reference signal thathas been precoded with different processing. In one embodiment, the eNBtransmits the precoded common sounding reference signals (PCSRS)periodically. The precoding may be based on certain predefined matricesselected by the eNB with the predefined matrices being environment orantenna configuration dependent. Alternatively, the precoding may bebased on initial full channel feedback provided by UE(s).

In an embodiment, the resource elements over which the PCSRS aretransmitted are located at predefined locations, in order to simplify UEdetection. For example, the PCSRS corresponding to different precodingmatrices can be transmitted cyclically according to a predefined patternin time and frequency. The PCSRS may be carried by a particular LTE-Aphysical resource block (PRB), for example, and information regardingthe location of the PRB as well as the cyclic pattern of the PCSRS maybe broadcasted to all UEs. In an embodiment, the eNB also precodes thePCSRS with a non-CoMP transmission precoding matrix (from a single cell)or a joint CoMP transmission precoding matrix (from multiple cells) toserve cell edge UEs or enable multi-cell MU-MIMO transmission. In anembodiment, the precoding is transparent to the cell edge UEs.

In step 210, the eNB receives CQI(s) from each UE. In an embodiment, theUEs measure the channel based on the PCSRS and reports the highest CQIor a specified number of the highest CQIs corresponding to certainprecoding processing. The UEs do not have knowledge precoding used inthe PCSRS in one embodiment. The UEs link the CQI with the correspondingprecoding by including a PCSRS index in its report of the CQI explicitlyor implicitly. For example, the UEs may report the location of the PRBused to receive the PCSRS or the cyclic pattern of the PCSRS. In afurther embodiment, the UE further links the reported CQI to a certainpredefined feedback channel.

In step 215, the eNB determines the best precoding matrix for each UEbased on the reported CQI(s). From the reported CQI(s) and the PCSRSindices from the different UEs, the eNB determines a precoding matrixfor each UE. In an embodiment, the eNB finds a best precoding index fora UE based on a one-to-one relationship between the precoding matrixindex and the CQI index. The CQI information may be further used by theeNB for the selection of a modulation and coding scheme (MCS) if thecorresponding precoding matrix is applied. In an embodiment, theinformation about the best precoding matrix and the corresponding CQIvalue may also be used by eNB to schedule MU-MIMO transmission.

FIG. 2b illustrates a flow diagram of embodiment UE method 250 for DLtransparent channel sounding. UE method 250 describes operations takingplace in a UE, such as UE 110, participating in DL transparent channelsounding to provide feedback information to an eNB, such as eNB 105, sothat the eNB can schedule CoMP transmissions.

Method 250 begins in step 255 with the UE measuring a downlink channelusing the PCSRS transmitted by the eNB. In step 260, the UE reports thehighest CQI or a specified number of the highest CQIs to the eNB. The UEdetermines where to make the measurements using the location of the PRBsor cyclic patterns broadcast by the eNB. In an embodiment, the UE doesnot need to know the precoding used in the PCSRS and links the CQI (themeasurement) with a corresponding precoding by including a PCSRS indexin its report to the eNB.

In order to optimize closed-loop (CL) performance for CoMP transmissionand advanced MIMO, sufficient spatial granularity may be needed in anembodiment. For UEs with medium to high mobility or for a situation withhigh correlated antennas, less spatial resolution requirement may beneeded in conditions where it is difficult to track a narrow beamdirection. However, rough (or coarse) beamforming may still provide ameasure of CL gain when compared with open-loop (OL) transmission. Thismay be especially true for CoMP transmission when compared with OL CoMPtransmission. Lower spatial resolution means fewer hypotheses and lowerPCSRS overhead in some embodiments.

In embodiments, a higher spatial resolution may be used for low mobilityUEs since to fine tune the beam formed beam. As an inverse to lowerspatial resolution, higher spatial resolution means more hypotheses andhigher sounding DL reference sequences. An efficient DL soundingapproach that enables a fast CL beamforming adaptation with reasonableDL sounding overhead is needed. Two possible solutions exist: iterativesounding and differential PMI feedback.

FIG. 3a illustrates diagram 300 of an embodiment iterative PCSRS basedDL channel sounding in advanced MIMO. Diagram 300 illustrates a portionof a communications system having eNB 305 and a number of UEs, such asUE 310, UE 315, and UE 316. UE 310 may be a UE with high mobility, whileUE 315 and UE 316 may be UEs with low mobility. In alternativeembodiments, greater or fewer UEs can be present.

In an embodiment, iterative PCSRS are applied to speed up the pollingprocedure for low mobility UEs during sounding. Iterative PCSRS usesrough tuning and fine tuning to reduce an overall number of hypotheses.In an embodiment, fine tuning is performed about a beam directiondetermined using rough tuning. This decreases PCSRS overhead and allowsfor fast CL adaptation.

In an embodiment, eNB 305 transmits a number of rough tuning PCSRSprecoded with processing matrices which separates the PCSRS roughlyequally in space as shown as solid ovals 325 and 330. The UEs measurethe rough tuning PCSRS and report back CQI(s), from which eNB 305determines that a rough tuning PCSRS corresponding to oval 325 isreported as highest CQI for UE 310. Similarly, for UEs 315 and 316, arough tuning PCSRS corresponding to oval 330 is reported as highest CQI.Since UE 310 is a high mobility UE, eNB 305 does not attempt to finetune to increase spatial resolution in some embodiments. UEs 315 and 316are low mobility UEs, to which eNB 305 increases spatial resolutionthrough fine tuning.

In an embodiment, eNB 305 achieves fine tuning by transmitting finetuning PCSRS precoding with processing matrices, which separates thefine tuning PCSRS about equally in space within a region encompassed byrough tuning PCSRS corresponding to oval 330 (shown as dotted ovals335-338). UEs 315 and 316 measure the fine tuning PCSRS and report backthe CQI(s), from which eNB 305 determines the fine tuning PCSRScorresponding to the reported CQI(s) from the UEs.

In an embodiment, fine tuning may be performed in several steps, witheach step obtaining greater and greater spatial resolution. In practice,the number of fine tuning steps can be determined by factors such as themobility of the UEs, the amount of time (and other resources) that canbe dedicated to the fine tuning, for example.

FIG. 3b illustrates an embodiment diagram 350 of iterative PCSRS basedDL channel sounding in CoMP transmission. As shown in FIG. 3b , diagram350 is similar to diagram 300 and the iterative PCSRS based DL channelsounding in CoMP transmission is substantially similar to the iterativePCSRS based DL channel sounding in advanced MIMO. A difference beingthat more than one eNB (eNBs 355 and 356 in FIG. 3b ) are used totransmit the PCSRS.

FIG. 4a illustrates a flow diagram of embodiment method 400 for PCSRSbased DL channel sounding in CoMP transmission. In an embodiment, method400 is performed, for example by an eNB, such as eNB 105 (FIG. 1), toobtain feedback information to schedule CoMP transmissions.

In an embodiment, the eNB transmits a rough tuning PCSRS that isprecoded with processing matrices that separate the rough tuning PCSRSroughly equally space in step 405. In an embodiment, rough beams allowfor identification with less spatial granularity. In step 407, the eNBreceives CQI reports from the UEs, which perform channel measurements ofthe rough tuning PCSRS and selects the strongest CQI (or a specifiednumber of the strongest CQI), and reports the CQI back to the eNB. In anembodiment, the reported CQI(s) corresponds to a particular precodingmatrix and corresponding PCSRS index.

From the reported CQI, the eNB determine a best PCSRS (and hence a bestbeam direction) for each UE in step 409. In step 411, the eNB uses thebeam direction as a baseline for CL precoding processing. In anembodiment, the beam direction may also be used as a fall back forprecoding processing when the eNB needs to override the recommendationsof the UEs, for example.

In step 413, the eNB transmit fine tuning PCSRS that have been precodedwith processing matrices that cover a region covered by rough tuningPCSRS selected by the UEs. If there are more than one rough tuning PCSRSto fine tune, each of the additional rough tuning PCSRS are fine tunedone at a time with additional fine tuning PCSRS. The eNB then receiveCQI reports from the UEs. In an embodiment, the UEs have perform channelmeasurements of the fine tuning PCSRS and select the strongest CQI (or aspecified number of the strongest CQI) and reports them back to the eNBin step 415. The fine tuning PCSRS are transmitted to the low mobilityUEs. In an embodiment, the eNB identifies the low mobility UEs needingadditional fine tuning, and informs the UEs or the UE in CL mode that itis in need of fine tuning. The reported CQI(s) correspond to aparticular precoding matrix and corresponding PCSRS index. From thereported CQI, the eNB determine the best fine tuning PCSRS (and hencethe best beam direction) for each UE in step 417.

As discussed previously, if there are multiple rough tuning PCSRS tofine tune, then the eNB may repeat the transmission of fine tuning PCSRSfor each of the rough tuning PCSRS. In an embodiment, the fine tuningPCSRS are specifically designed for each of the rough tuning PCSRS. In afurther embodiment, the fine tuning step can be performed multiple timesto obtain a progressively finer and finer spatial resolution. In anotherembodiment, the fine tuning for more than one rough tuning beams can beperformed at the same time.

FIG. 4b illustrates a flow diagram of UE method 450 for PCSRS based DLchannel sounding in CoMP transmission. In an embodiment, method 450 isused by a UE such as UE no (FIG. 1), participating in PCSRS based DLchannel sounding to provide feedback information to an eNB, such as eNB105, so that the eNB can schedule CoMP transmissions.

In step 455, the UE measures a downlink channel using the coarse tuningPCSRS transmitted by the eNB. The UE then reports the highest measuredCQI or a specified number of highest CQI in step 457. In an embodiment,method 450 continues with the UE measuring the downlink channel usingthe fine tuning PCSRS transmitted by the eNB in step 459. The UE thenreports the highest measured CQI or a specified number of highest CQI instep 461.

In an embodiment, if there are several PCSRS that have a same highestmeasured CQI, then the UE reports all of the indices. Alternatively, theUE selects a specified number of the indices to report. In a furtherembodiment, the UE selects one index to report. The selection of theindex (or indices) may be performed, based on eNB operating conditions(load, number of UEs served, UE priority, for example).

As discussed previously, in some embodiments, not all UEs participate inthe measurement of the downlink channel with the fine tuning PCSRS. TheUEs may receive messages from the eNB requesting that they participatein the fine tuning step. Alternatively, the UEs operate in CL mode andparticipate in the fine tune step.

In an embodiment, differential PMI feedback combines PMI feedback withPCSRS DL channel sounding. A differential codebook is applied to furtherimprove the precoding accuracy of PCSRS based rough beam tuning. In anembodiment, the differential codebook is used to enhance the spatialresolution of a base codebook with the same codebook size, which reducesthe codebook search space. In an embodiment, the differential codebookis used to trace the change of the channel. Here, the differentialcodebook search uses the precoded demodulation reference sequence as areference.

FIG. 5a illustrates a flow diagram of an embodiment base station method500 for PCSRS based differential codebook feedback for. In anembodiment, method 500 is performed by an eNB such as eNB 105, to obtainfeedback information to schedule CoMP transmissions.

In an embodiment, the eNB initiates an initial channel sounding bysending PCSRS precoded with eNB selected precoding matrices in step 505.According to an embodiment, the precoding matrices are selected withlarge granularity. The eNB receives the indices of PCSRS having highestmeasured CQI as well as the CQI value itself from the UEs in step 507.

In addition to the PCSRS indices and CQI, the eNB also receives a PMIfrom a differential codebook search performed by the UEs in step 509.The eNB then determines a precoding matrix based on the reported PMI andthe received PCSRS index and CQI in step 511. In an embodiment, the eNBuses the reported PCSRS from the UEs to verify the PMI feedback.

In an embodiment, the eNB overrides the PMI recommendation from the UEs(step 509) with a precoding matrix that it computes on its own from thePCSRS indices and CQI received from the UEs.

FIG. 5b illustrates embodiment UE method 550 for PCSRS baseddifferential codebook feedback. In an embodiment, method 550 isperformed by a UE, such as UE no, to provide feedback information to aneNB, such as eNB 105, so that the eNB can schedule advanced MIMOtransmission or CoMP transmissions.

In step 555, the UE measures a DL channel using the PCSRS transmitted bythe eNB, where the PCSRS has been precoded with precoding matrices. Inan embodiment, the precoding matrices have large granularity. In step557, the UE report an index of a PCSRS corresponding to a highestmeasured CQI to the eNB. In a further embodiment, the UE may also reportthe highest measured CQI value in addition to the index of the PCSRS.

In step 559, the UE narrows down the codebook search by performing adifferential codebook search along a direction of the PCSRS having thehighest measured CQI. The PCSRS that resulted in the highest measuredCQI may be used by the UE as a reference in the differential codebooksearch. In some embodiments, only UEs participating in CoMPtransmissions perform the differential codebook search, where the UEsare based on DL signaling or a present CQI threshold. In otherembodiments, UEs not participating in CoMP transmission may also performthe differential codebook search to provide more precise precodinginformation for MU-MIMO transmission. In step 561, the UE reports to theeNB a best PMI from the differential codebook search.

In an embodiment, if there are several PCSRS that have the same highestmeasured CQI, the UE reports all of the indices. Alternatively, the UEmay select a specified number of the indices to report, or the UE mayselect one index to report. In an alternative embodiment, the selectionof the index (or indices) is performed based on eNB operating conditionsincluding, but not limited to load, number of UEs served, and UEpriority.

FIG. 6 illustrates a diagram 600 of an embodiment PCSRS based channelsounding for CS/CB. Diagram 600 illustrates a portion of acommunications system having a first eNB 605 and a second eNB 610. FirsteNB 605 serves UE 615, while second eNB 610 serves UE 616. The systemshown in FIG. 6 is one example of many possible configurations. Inalternative embodiments, the system illustrated in FIG. 6 can havegreater or fewer UEs, eNBs and beams.

Based on measurements of PCSRS precoded with different precodingmatrices transmitted by first eNB 605 and second eNB 610, UE 616 reportsback to both eNBs PCSRS indices corresponding to a highest measured CQIor both a highest and a lowest measured CQI. In other words, UE 616reports back to the eNBs the strongest and weakest beam directions. Inone example, dashed oval 620 represents a weakest beam direction fromfirst eNB 605 and solid oval 625 represents a strongest beam directionfrom second eNB 610. Using the information provided by the UEs, the eNBsschedule transmissions to its own UEs. For example, with knowledge ofthe weakest beam direction from first eNB 605 and the strongest beamdirection from second eNB 610 from UE 616, first eNB 605 schedulestransmissions to its UE 615 which causes the least interference to UE616 and second eNB 610 schedules transmissions to UE 616 in thestrongest beam direction 625 at the same time that first eNB 605 istransmitting to UE 615.

FIG. 7a illustrates a flow diagram of embodiment eNB method 700 forPCSRS based DL channel sounding for CS/CB. In am embodiment, method 700is performed by an eNB, such as eNB 105, performing PCSRS based DLchannel sounding for CS/CB.

In step 705, the eNB transmits a PCSRS precoded with different precodingmatrices. The eNB then receives an index of PCSRS corresponding to PCSRShaving a highest measured CQI or indices of PCSRS having highest andlowest measured CQI in step 707. The eNB informs neighboring cells(eNBs) of weakest beam directions of its cell edge UEs in step 709. Instep 711, the eNB schedules transmission to UEs in a same beam directionas weakest beam direction of neighboring cells.

FIG. 7b illustrates a flow diagram of UE method 750 for PCSRS based DLchannel sounding for CS/CB. In an embodiment, method 750 is performed bya UE, such as UE no, participating in PCSRS based DL channel soundingfor CS/CB.

In step 755, the UE measures a DL channel using the PCSRS transmitted bythe eNB. The UE then reports either an index of a PCSRS having highestmeasured CQI or indices of PCSRS having highest measured CQI and lowestmeasured CQI in step 757.

In an embodiment, if there are several PCSRS that have the same highest(or lowest) measured CQI, the UE may report all of the indices.Alternatively, the UE may select a specified number of the indices toreport, or the UE may select one index to report. In a furtherembodiment, the selection of the index (or indices) may be performedbased on eNB operating conditions including, but not limited to load,number of UEs served and UE priority.

In some embodiment, there may be different CoMP transmission feedbackschemes for backhaul CoMP transmission and access link CoMPtransmission. For example, an explicit feedback scheme may be used forbackhaul CoMP transmission. With backhaul CoMP transmissions to fixedRNs, an explicit feedback scheme may have an acceptable level of ULfeedback overhead.

In an embodiment, a hybrid DL channel sounding scheme is used for accesslink CoMP transmission. The hybrid DL channel sounding scheme includesboth explicit and implicit DL channel sounding. The explicit DL channelsounding is performed as an initial DL channel sounding with the UEsfeeding back information regarding the DL channel to the serving eNB.For CoMP transmission, the UE feeds back the DL channel to theneighboring eNBs as well. In an embodiment, implicit channel sounding isused to keep track of changes in the DL channel. Additionally, anexplicit feedback scheme may be used for fixed UEs, while a PCSRS basedfeedback scheme may be used for UEs that are mobile having low, mediumor high mobility. In an embodiment, the PCSRS based feedback scheme usesnon-adaptive PCSRS based sounding for medium and high mobility UEs andadaptive PCSRS based sounding and PCSRS based differential codebookfeedback for low mobility UEs.

In further embodiment, hybrid DL channel sounding schemes are used forhigher-order SU-MIMO or MU-MIMO (referred to collectively as advancedMIMO). In higher-order MIMO systems, non-adaptive techniques, such asnon-adaptive PCSRS based sounding, are used for medium and high mobilityUEs or for a system having highly correlated antennas. In an embodiment,adaptive PCSRS based sounding is used for low mobility UEs. Inhigh-order MIMO systems with uncorrelated transmit antennas; a PCSRSbased differential codebook feedback technique can be used.

A block diagram of an embodiment base station 800 is illustrated in FIG.8. Base station 800 has base station processor 804 coupled totransmitter 806 and receiver 808, and network interface 802. Transmitter806 and receiver 808 are coupled to antenna 812 via coupler 810. Basestation processor 804 executes embodiment methods and algorithms. In anembodiment, base station 800 is configured to operate in a LTE networkusing an OFDMA downlink channel divided into multiple subbands and usingsingle carrier FDMA in the uplink. In alternative embodiments, othersystems, network types and transmission schemes can be used, forexample, 1×EV-DO, IEEE 802.11, IEEE 802.15 and IEEE 802.16. Inalternative embodiments, base station 800 can have multipletransmitters, receivers and antennas (not shown) to support MIMOoperation.

A block diagram of an embodiment relay node 900 is shown in FIG. 9.Relay node 900 has donor antenna 920, which transmits to and from thebase station and is coupled to coupler 918, transmitter 922 and receiver916. Service antenna 912, which transmits to and receives signals fromuser devices, is coupled to coupler 910, transmitter 906 and receiver908. RN processor 914, which is coupled to both the donor and servicesignal paths, controls the operation of relay node and implementsembodiment algorithms described herein. In an embodiment of the presentinvention, relay node 900 is configured to operate in a LTE networkusing an OFDMA downlink channel divided into multiple subbands and usingsingle carrier FDMA in the uplink. In alternative embodiments, othersystems, network types and transmission schemes can be used.

A block diagram of embodiment user device 1000 is illustrated in FIG.10. User device 1000 can be, for example, a cellular telephone or othermobile communication device, such as a computer or network enabledperipheral. Alternatively, user device 1000 can be a non-mobile device,such as a desktop computer with wireless network connectivity. Userdevice 1000 has mobile processor 1004, transmitter 1006 and receiver1008, which are coupled to antenna 1012 via coupler 1010. User interface1002 is coupled to mobile processor 1004 and provides interfaces toloudspeaker 1014, microphone 1016 and display 1018, for example.Alternatively, user device 1000 may have a different configuration withrespect to user interface 1002, or user interface 1002 may be omittedentirely. In embodiment, user device is configured to operate accordingto embodiment algorithms. In alternative embodiments, user device 1000can have multiple transmitters, receivers and antennas (not shown) tosupport MIMO operation.

In an embodiment, a method of operating a base station configured tocommunicate with at least one user device includes transmitting areference signal to the at least one user device, receiving channelquality information from the at least one user device, and forming abeam based on the channel quality information received from the at leastone user device. In one embodiment forming the beam comprises forming abeam using a plurality of antennas. In a further embodiment, forming thebeam includes coordinating a transmission from at least one further basestation. In an embodiment, coordinating the transmission comprisescommunicating the at least one further base station via an X2 link. Insome embodiment, forming a beam includes computing a precoding matrixfor the at least one user device, and in a further embodiment, formingthe beam includes iteratively adjusting the beam based on the channelquality information from the at least one user device. In an embodiment,the reference signal is a precoded common sounding reference signal(PCSRS).

In an embodiment, the method also includes sharing weakest beamdirections with the at least one further base station, and schedulingtransmissions to the at least one user device using precoding matricescorresponding to weakest beam directions of the least one further basestation. In an embodiment, sharing weakest beam directions furtherincludes transmitting weakest beam directions the at least one furtherbase station, and receiving weakest beam directions from the at leastone further base station.

In some embodiment, the base station is operated on a long termevolution (LTE) network. The base station is an eNB and the at least oneuser device comprises a UE. Alternatively, the base station can be anRN.

In a further embodiment, forming the beam includes performing a coarsebeam adjustment based on the channel quality information from the atleast one user device. After the coarse beam adjustment, a fine beamadjustment is performed. The fine beam adjustment includes receivingprecoding matrix indication (PMI) based differential codebook feedbackfrom the at least one user device and adjusting the beam based on thePMI differential codebook feedback.

In a further embodiment, a method for operating in a communicationsnetwork having a plurality of communications devices includes performinga coarse tuning with the plurality of communications devices to achievea first degree of spatial granularity; and performing a fine tuning witha subset of the plurality of communications devices to achieve a seconddegree of spatial granularity. In an embodiment, the subset ofcommunications devices includes communications devices having lowmobility. In one embodiment, performing the coarse tuning includestransmitting a first number of precoded reference sequences to theplurality of communications devices, receiving channel information fromthe plurality of communications devices, and computing a best rough tuneprecoding matrix for each communications device based on the receivedchannel information. In an embodiment, the precoded reference sequencesare precoded with precoding matrices that roughly separate the precodedreference sequences about equally in space.

In an embodiment, the received channel information includes channelquality measurements for each of the precoded reference sequences. Thereceived channel information includes a highest channel qualitymeasurement for a precoded reference sequence out of the number ofprecoded reference sequences in an embodiment. In some embodiments, thebest rough tune precoding matrix includes a precoding matrix used toprecode a precoded reference sequence corresponding to the highestchannel quality measurement.

In an embodiment, performing the fine tuning includes selecting a bestprecoding matrix based on results of the coarse tuning and transmittinga second number of precoded reference sequences to the subset ofcommunications devices, where the precoded reference sequences areprecoded with precoding matrices that separate the precoded referencesequences about equally around a beam direction corresponding to thebest precoding matrix. The method also includes receiving second channelinformation from the subset of communications devices and computing abest fine tune precoding matrix for each communications device in thesubset of communications devices based on the received second channelinformation. In one embodiment, there are multiple best precodingmatrices, and the method further includes repeating the selecting, thetransmitting, the receiving, and the computing for each best precodingmatrix in the multiple best precoding matrices.

In an embodiment, a base station includes an antenna, a transmittercoupled to the antenna, a receiver configured to receive channel qualityinformation from at least one user device, and a processor calculating abeam direction for the least one user device, the beam direction basedon the received channel quality information. The transmitter isconfigured to transmit a sounding signal to the at least one userdevice. In an embodiment, the antenna includes a plurality of antennasand the transmitter transmits via the plurality of antennas usingmultiple input, multiple output (MIMO) techniques. In a furtherembodiment, the base station further includes a communication interfacewith at least one further base station, where the base stationcoordinates transmission with the at least one further base stationusing the communication interface.

In an embodiment, a relay node (RN) includes a backhaul link transceiverand an access link transceiver. The relay node receives firstcoordinated multi-point (CoMP) transmissions from a plurality of basestations from the backhaul link transceiver, and the relay nodetransmits second CoMP transmissions on the access link transceiver to atleast one user device in cooperation with at least one further basestation or at least one further relay node. In an embodiment, the firstCoMP transmissions have a different feedback scheme from the second CoMPtransmissions.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed is:
 1. A method of operating a base station configuredto communicate wirelessly in a wireless network, the method comprising:transmitting, by the base station, precoded reference signals to a userdevice over different beam directions; receiving, by the base station,at least one index from the user device, the at least one index uniquelyidentifying at least one of the precoded reference signals transmittedto the user device; and transmitting, by the base station, a beam-formeddata signal to the user device, wherein transmitting the beam-formeddata signal comprises setting a beam direction of the beam-formed datasignal based on the at least one index received from the user device. 2.The method of claim 1, wherein the beam-formed data signal istransmitted from a single base station.
 3. The method of claim 1,wherein the beam-formed data signal is a joint transmission coordinatedbetween multiple base stations.
 4. The method of claim 3, furthercomprising: sharing a first weakest beam direction between two of themultiple base stations; and scheduling transmissions to the user deviceusing precoding matrices corresponding to the first weakest beamdirection.
 5. The method of claim 3, wherein control signaling forcoordinating the joint transmission is communicated between at least twoof the multiple base stations via an X2 link.
 6. The method of claim 1,wherein the beam-formed data signal is a joint transmission coordinatedbetween the base station and at least one relay node on a backhaul linkand on an access link.
 7. The method of claim 1, wherein the wirelessnetwork is a long term evolution (LTE) network, the base stationcomprises an enhanced Node B (eNB), and the user device comprises userequipment (UE).
 8. The method of claim 1, wherein transmitting abeam-formed data signal to the user device comprises computing aprecoding matrix for the user device.
 9. The method of claim 1, whereintransmitting the beam-formed data signal to the user device comprises:performing a coarse beam adjustment based on the at least one indexreceived from the user device; and performing a fine beam adjustmentafter performing the coarse beam adjustment.
 10. The method of claim 9,wherein performing the fine beam adjustment comprises: receivingprecoding matrix indication (PMI) differential codebook feedback fromthe user device, and adjusting the beam direction in accordance with thePMI differential codebook feedback.
 11. The method of claim 1, furthercomprising: performing a coarse tuning with the user device to achieve afirst degree of spatial granularity; and performing a fine tuning withthe user device to achieve a second degree of spatial granularity. 12.The method of claim 11, wherein performing the coarse tuning comprises:transmitting precoded reference sequences to the user device, whereinthe precoded reference sequences are precoded with precoding matricesseparating the precoded reference sequences; receiving channelinformation related to the from the user device; and computing a bestrough tune precoding matrix for the user device based on the receivedchannel information.
 13. The method of claim 12, wherein the receivedchannel information comprises channel quality measurements for each ofthe precoded reference sequences.
 14. The method of claim 12, whereinthe received channel information comprises a highest channel qualitymeasurement for a precoded reference sequence out of the precodedreference sequences.
 15. The method of claim 14, further comprisinggenerating one or more additional precoded reference sequences based onthe best rough tune precoding matrix, wherein the best rough tuneprecoding matrix corresponds to the precoded reference sequence havingthe highest channel quality measurement.
 16. A base station in awireless network, the base station comprising: a processor; and anon-transitory computer readable storage medium storing programming forexecution by the processor, the programming including instructions to:transmit precoded reference signals to a user device over different beamdirections; receive at least one index from the user device, the atleast one index uniquely identifying at least one of the precodedreference signals transmitted to the user device; and transmit abeam-formed data signal to the user device, wherein the instructions totransmit the beam-formed data signal include instructions to set a beamdirection of the beam-formed data signal based on the at least one indexreceived from the user device.
 17. The base station of claim 16, whereinthe beam-formed data signal is transmitted from a single base station.18. The base station of claim 16, wherein the beam-formed data signal isa joint transmission coordinated between multiple base stations.
 19. Thebase station of claim 18, wherein the programing further includesinstructions to: share a first weakest beam direction between two of themultiple base stations; and schedule transmissions to the user deviceusing precoding matrices corresponding to the first weakest beamdirection.
 20. The base station of claim 18, wherein control signalingfor coordinating the joint transmission is communicated between at leasttwo of the multiple base stations via an X2 link.
 21. A method ofoperating a user device configured to communicate wirelessly in awireless network, the method comprising: receiving, by the user device,precoded reference signals from a base station; transmitting, by theuser device, at least one index to the base station, the at least oneindex uniquely identifying at least one of the precoded referencesignals received from the base station; and receiving, by the userdevice, a beam-formed data signal from the base station.
 22. The methodof claim 21, wherein the beam-formed data signal is received from asingle base station.
 23. The method of claim 21, wherein the beam-formeddata signal is a joint transmission coordinated between multiple basestations.
 24. The method of claim 23, wherein control signaling forcoordinating the joint transmission is communicated between at least twoof the multiple base stations via an X2 link.
 25. The method of claim23, wherein the beam-formed data signal is a joint transmissioncoordinated between the base station and at least one relay node on abackhaul link and on an access link.
 26. The method of claim 21, whereinthe wireless network is a long term evolution (LTE) network, the basestation comprises an enhanced Node B (eNB), and the user devicecomprises user equipment (UE).
 27. The method of claim 21, whereinfurther comprising: receiving precoded reference sequences from the basestation, wherein the precoded reference sequences are precoded withprecoding matrices separating the precoded reference sequences;estimating channel information based on the precoded referencesequences; and transmitting the channel information to the base station.28. The method of claim 27, wherein the channel information compriseschannel quality measurements for each of the precoded referencesequences.
 29. The method of claim 27, wherein the channel informationcomprises a highest channel quality measurement for a precoded referencesequence out of the precoded reference sequences.
 30. A user device in awireless network, the user device comprising: a processor; and anon-transitory computer readable storage medium storing programming forexecution by the processor, the programming including instructions to:receive precoded reference signals from a base station; transmit atleast one index to the base station, the at least one index uniquelyidentifying at least one of the precoded reference signals received fromthe base station; and receive a beam-formed data signal from the basestation.
 31. The user device of claim 30, wherein the beam-formed datasignal is received from a single base station.
 32. The user device ofclaim 30, wherein the beam-formed data signal is a joint transmissioncoordinated between multiple base stations.
 33. The user device of claim32, wherein control signaling for coordinating the joint transmission iscommunicated between at least two of the multiple base stations via anX2 link.
 34. The user device of claim 32, wherein the beam-formed datasignal is a joint transmission coordinated between the base station andat least one relay node on a backhaul link and on an access link. 35.The user device of claim 30, wherein the wireless network is a long termevolution (LTE) network, the base station comprises an enhanced Node B(eNB), and the user device comprises user equipment (UE).
 36. The userdevice of claim 30, wherein the programming further includesinstructions to: receive precoded reference sequences from the basestation, wherein the precoded reference sequences are precoded withprecoding matrices separating the precoded reference sequences; estimatechannel information based on the precoded reference sequences; andtransmit the channel information to the base station.
 37. The userdevice of claim 36, wherein the channel information comprises channelquality measurements for each of the precoded reference sequences.